In wireless communications, different frequency bands are set aside by the Federal Communications Commission (FCC) for different purposes. Users of a particular frequency band may be primary or secondary, licensed or unlicensed users. Existing users of a particular frequency spectrum are called incumbents of that spectrum. A Cognitive Radio (CR) is typically an unlicensed secondary device that relies on dynamic spectrum access techniques to gain access to spectrum that is used by incumbents (e.g., primary licensed users). For example, in the Television White Space (TVWS) spectrum, typical incumbents include television and wireless microphone transmitters. Other licensed secondary devices include public safety radio operators.
Cognitive Radios operate on secondary channels—frequencies that may be used intermittently by the primary/licensed devices. Regulatory bodies such as the FCC are in the process of establishing rules governing the use of spectrum by secondary devices. As the use of secondary channels is permissive and is reserved for higher priority communications, any Cognitive Radios operating on such a secondary channel must monitor the channel for communications from primary devices and change transmission and reception parameters to avoid interfering with these communications.
Sensing of the active communications channel by a Cognitive Radio is performed periodically using “a quiet period.” This is a period of time in which all Cognitive Radio system communications on the channel cease, thereby allowing Cognitive Radios to sense for higher priority/incumbent signals in a radio-quiet environment. If a higher priority signal, e.g., a licensed or unlicensed narrowband wireless audio equipment signal such as wireless microphone signal in U.S.TVWS spectrum is detected, under the current FCC rules, the Cognitive Radio must vacate the channel.
However, once a communications channel has been selected by the Cognitive Radio, it may be desirable for the Cognitive Radio network to retain the selected channel due to limited available spectrum. This may be especially problematic in an urban environment, where there may only be a few channels available for use. As there are a large number of heterogeneous secondary systems that may attempt to capture the occupied channel during a quiet period, the resulting determination of which system may use the channel is inefficient. In one example, during the quiet period, any signal or emission that looks like one or more narrowband (200 KHz) signals is given priority and will capture the channel, even if the previous user actually has priority.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments shown so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Other elements, such as those known to one of skill in the art, may thus be present.